STUDY OF THE STRUCTURE AND PHASE-BEHAVIOR OF DIPALMITOYLPHOSPHATIDYLCHOLINE BY INFRARED-SPECTROSCOPY - CHARACTERIZATION OF THE PRETRANSITION AND SUBTRANSITION

Despite the fact that fully hydrated multilayers of phosphatidylcholin es have been extensively studied in the past, phase transitions such a s the pretransition and the subtransition are still the object of diff erent interpretations and contradictory observations. In order to obta in more information on the structural changes associated to these tran sitions for dipalmitoyl phosphatidylcholine (DPPC), a study has been d one using both transmission and polarized attenuated total reflection infrared spectroscopy. At the pretransition, we observe that the exten ded hydrocarbon chains become less tilted with respect to the bilayer normal. The pretransition is also characterized by an increase in the hydration of the DPPC bilayer and the appeareance of the ripple struct ure. As opposed to previous findings, we do not observe a major variat ion of the acyl chain packing mode at the pretransition. These observa tions are supported by the investigation of DPPC with 5 mol% cholester ol, since this sterol is known to be a strong inhibitor of the pretran sition. On the other hand, a drastic change in chain packing, characte rized by infrared bands due to the methylene scissoring mode, is obser ved at the subtransition. Also, below the subtransition, we observe an increase of the hydrocarbon chain rotational disorder. Some correlati ons have also been made between the spectral parameters normally used to characterize the degree of order of the acyl chains. These correlat ions have allowed us to evaluate more precisely at the molecular level the subtle structural changes tl-lat occur during phase transitions o f DPPC. (C) 1998 Elsevier Science Ireland Ltd. All rights reserved.